1. Technical Field
The present invention pertains to power supplies, and more particularly, to a power supply for use with a subscriber's telephone instrument that includes one or more memory devices which need a continuous current source so as to maintain data stored within the memories.
2. Background Art
The concept of utilizing the telephone line, as connected to a central switching point, as a power source to both maintain (while in the on-hook position) and manipulate (while in the off-hook position) the data contents of a memory device has been implemented in telephones equipped with pulse dialers including the so-called redial feature, such as implemented in the Flip Phone.RTM. II manufactured by GTE Business Communications Systems Incorporated.
Referring to FIG. 1 of the prior art as shown, a resistance in excess of ten megohms consisting of resistors R1 and R2 is connected between the output of the diode bridge polarity guard B1 which interfaces to the telephone line and the power supply input (V+) to the pulse dialer IC-1. Connected in parallel with the pulse dialer IC-1 is a small storage capacitor C1 which is present to maintain the data contents stored in the pulse dialers memory circuitry, during momentary interruptions in the DC voltage across the telephone line which is connected to those conductors designated tip and ring. Prior to the so-called avalanche breakdown of the integrated circuit IC-1, which nominally occurs at about 12 volts, the idle load presented by the integrated circuit IC-1 approximates that of a one megohm resistor. Given an on-hook line voltage of 50 volts DC, these two resistances form a voltage divider preventing any loss of memory due to avalanche breakdown. Since the associated logic circuitry, included in the integrated circuit IC-1, is designed to share the same power supply, no conflict exists between the signal interface of the memory and logic circuitry. A constant current is supplied to operate the integrated circuit IC-1 in the off-hook mode.
In a number of the telephones currently available which include a number of different features as well as a repertory dialer, such as, the "Execuphone," manufactured by TIE Incorporated, and the "Duophone 160," marketed by Radio Shack Incorporated, which utilize a CMOS microprocessor; a random access memory (RAM) and dual tone multifrequency tone dialer, both implemented as integrated circuits, perform the necessary control functions. Generally, these integrated circuit units are powered by a five volt battery supply derived from either the AC power line, the telephone line, or a backup internally-included battery. Data retention by the CMOS random access memory is maintained in the on-hook condition by either the AC power line or the backup battery.
Some subscribers' telephones which incorporate a multitude of features, including that of repertory dialing, such as that manufactured by American Telecommunications Corporation under the name "Hangtel," may also be completely powered by the telephone line. The circuit shown in prior art FIG. 2 is somewhat similar to the pulse dialer circuit of prior art FIG. 1. However, in FIG. 2 the random access memory quiescent or memory retention current represents a leakage current effect rather than that of a resistive load. A 5.6 volt zener diode CR3 is utilized as a clamp, which prevents the random access memory IC-2 from entering avalanche breakdown. Typically, the zener diode clamps the random access memory's IC-2 supply voltage to 4.9 volts given an idle telephone line voltage of 50 volts. Since the voltage supply for the microprocessor IC-3 is set by 5.1 volt zener diode CR6, the S1 input to the microprocessor will not experience any ill-effects due to a high level voltage from the random access memory IC-2's DO output Such an event, however, could change the internal logic states of the microprocessor. Without such compatibility between the on-hook voltage supply for the random access memory and the off-hook voltage supply for the microprocessor, the storage capacitor C2 would prolong any excess voltage transient appearing at the input to the microprocessor. This could only further degrade the integrity of the microprocessor IC-3.
The principal disadvantage of the prior art circuit of FIG. 2 is that the clamping zener supplied to the random access memory must exhibit a sharp breakdown knee in its current versus voltage characteristics. Otherwise, the clamping zener would drain the current needed by the random access memory to maintain greater than the minimum data retention supply voltage. Such a requirement forces the microprocessor to operate with a voltage supply in the neighborhood of 5 volts. Unfortunately, telephones connected to long subscriber loops may not be able to supply this voltage level from the telephone line. It should be noted that Electronic Industry Association (EIA) specification RS-470 specifies that the input voltage to the telephone must not exceed 6 volts given a loop current of 20 milliamps in the off-hook state.
Accordingly, it is the object of the present invention to provide a power supply for use with a subscriber's telephone, which includes a repertory dialer, which may be completely powered from the telephone subscriber's line. The design of the power supply is such as to overcome the objections to prior art power supplies, while being compatible with both the on and off-hook states of the subscriber's telephone set.